If you decide to tell him, I would consider going through "official" channels to do it. Maybe the chaplain's office. I think his superiors need to know that he will be dealing with this while he's there. I know for other sorts of things (births, deaths, serious illness, etc.), you can go through the American Red Cross, but I doubt it would be appropriate in this situation; you could give them a call to see. If a message goes through AMCROSS it always goes through the chain of command and through the chaplain's office. So he would be getting some support while dealing with the information. If you don't go through his chain of command to deliver this news, please encourage him to tell his drill sargeant and/or commanding officer so they are aware.
If you need help figuring out who to contact, should you decide to go the official route, PM me and I'll see what I can find out.